Method of shrinking magnesia



L. E SAUNDERS.

METHOD OF SHRINKINVG MAGNESIA.

APPLICATION FILED JULY28. I917. RENEWED FEB. 17.1920

1 352, 388. Patented Sept. 7, 1920.

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LEWIS E. SAUNDERS, OF WOmmTEB, MASSACHUSETTS, ASSIGNOB TO NORTON COM- run, or wolm'mn, MASSACHUSETTS, A CORPORATION OF MASSACHUSETTS.

METHOD OI SHBINKING. MAGNESIA.

Specification of Letters 2min.

Patented Sept. 7, 1920.

Application fled July 28, 1917, Serial 80. 188,387. Renewed February 17, 1920. Serial No. 359,818.

To all whom it may concern:

Beit known that I, Lawrs E. SAUNDERS,

a citizen of the United States, residing at Worcester, in the county of Worcester, and State of Massachusetts, have invented certain new and useful Improvements in Methods of Shrinking Magnesia, of whichthe following is a specification.

This invention is an electric-furnace method of heating magnesia.

It is well known that magnesia, when subjected to sufliciently high tem eratures, progressively increases in speci c gravity and decreases in volume, this being commonl termed shrinking. At sufficlently hig tem ratures, the magnesia" is sintered or' agg omerated into a solid-mass, and at temperatures easily attainable in the electric furnace, may be fused. This invention relates to an electric-furnace method by'which of pure ommerclal a somewhat lower temwhereas v at ordinary temperatures, it ispractically non-conductive. This invention contemplates establishing a current-path through a charge of magnesia by suitable application-of heat along a restricted line or lines, and thereafter electrically heatin and converting the'surrounding portion 0 the charge b means of an electric current traversing those portions of the char e which have been thus rendered electrical y conductive. a

The method ma be carried into effect in a furnace of the c aracte r illustrated in the accompanying drawing, wherein- Figure 1 is a horizontal section of the furnace on line 1-1 of Fig. 2,

F i 2 is a central longitudinal section on line I.II of Fig. 1; and

Fig. 3 is a resistance curve under specific conditions, as described below.

The furnace illustrated has side walls 1, 1, end walls 2, 2, and a base 3, all of which may be of fire-brick or other suitable refractory material, preferably basic in character. Magnesia brick may be used. The

terminal electrodes 4, 4 are of carbon and as indicated at 5, this facing preferably having a step-like formation, as shown, for the purpose of supporting those portions of the charge which are .in contact with the terminalelectrodes. The carbon rod 6 termed the starting rod of relatively small diameter'is' illustrated as-extending between. the electrodes and contacting therewith. This starting rod may if desired consist of two or more sections which are initially in electrical contact with each other, and therefore afford a continuous pathfor the current between the terminals 4, 4.

The method as carried out in a simple furnace of this kind is as follows: The magnesia, preferably calcined, is charged into the furnace completely filling the same and being packed under and around rod 6 and in close contact with the terminal electrodes 4, 4. A heavy current is then applied, which in the first instance, traverses the carbon rod 6. At the beginning of the operation,

the amperage and the voltage of the current are so adjusted with reference to the crosssection of the rod 6, that the portion of the charge immediately surrounding the rod is caused to agglomerate andto become elec- -trically conductive. At the high temperatures thus developed in the rod, the carbon disappears completely in the course of a very few minutes, being presumably oxid- 1ze sium being reoxidiz'ed to magnesia in the course of its passage through the charge.

'- a rapid rate as the carbon rod disappears in order properly to maintain the load.

The resistance curve, Fig. 3, will serve to illustrate the changes occurring in a fur nace during the course of a particular run,

by the magnesia, the reduced magne-- it being understood, howevcr, ithat suchfluctuations are here indicated are not necessarily characteristic ofthe'practice of the method. In the curve, the solid line represents the actual fluctuations of the resistance, and the dotted line represents a simplified expression of the changes in resistance. In the run referredtoaitwo parallel carbon rods were used, each three-fourths inch in diameter and eighty-eight inches long, each rod consisting of two lengths joined by a sleeve midway between the electrodes. The interior length of the furnace was 94 inches, its width 30 inches, and 1ts at the end of four minutes.

then rapidly increased over a period of about depth 37-}- inches. The total charge, including additions during the operation, consisted of about 3500 pounds'of a commercial calcined'magnesia. The initial current was 365 amperes at 50, volts, and was increased by short stages to 2330 amperes at 109, volts The resistance 16 minutes, during which'the voltage was progressively increased so as approximately to maintain the load, the voltage afterf20 minutes being 190 and the amperes 1460. The resistance then gradually declined, subject to fluctuations, the conditions ultimately becoming approximately constant at 131 volts and 2,000 to 2,100 amperes.

lt will be observed that this particular curve indicates three well-defined stages of the operation. It may be assumed that the rapid fall in resistance from A to 15 cor responds to the increase of conductivity of the carbon rod under the rapidly rising temmasses and in part to the presence of molten magnesia.

At the conclusion of the run,,after cooling the furnace, the magnesia was found in'the form of a sintered mass or pig weighing about 1900 ounds and bedded in the unconverted portions of the charge. The latter portions bein unsintered, were easily brushed off, an were immediately available, with the necessary additions, for a repetition of the operation. The pig consisted of fully shrunk material'having a specific gravity of 3.547. No vestige of the carbon rod remained.

During the course of the run, the charge decreases gradually in volume, and is preferably maintained by adding further portionsof magnesia as required. Such additions under ordinary conditions may amount approximately to the weight of i the charge originally taken.

It is of course to be understood that the invention is not restricted to such details of I establish a current path therethrough, and

progressively shrinking further portions of said charge by means of an electric current traversing the conductive portions thereof.

2. The electrical method of shrinking nesia in the body of a substantially nonconductive charge of magnesia, and progressively shrinking said charge by means of heat developed by an electric current traversing the conductive portions thereof.

Tn testimony whereof, I affix my signature.

LEWIS E. SAUN ERS;

magnesia, which consists in establishing a current path of electrically-conductive mag- 

